Brake discs for aircraft or automobiles require materials having high heat resistance and long wear. Asbestos has been used in these applications, due to its heat resistance properties. Asbestos-based friction components have limited applicability under severe use conditions because the polymeric resins used to bind the asbestos fibers together decompose at elevated temperatures. The use of carbon-carbon brake components has therefore become more common.
Among the types of substrates used to make carbon-carbon parts are discontinuous carbon fiber moldings, nonwoven airlaid carbon substrates, woven carbon fiber substrates, and braided carbon fiber substrates. Carbon-carbon parts can also be made by using fibers that are precursors to carbon fibers to build a preform and then converting the fiber to carbon fiber. The substrates are typically stacked on top of each other to a desired thickness, and then the stacked substrates may be needle-punched together to join or consolidate the substrates to each other by intermingling fibers between the layers of substrates to form the preform. Other methods of manufacturing a preform or impregnating the preform with pitch may also be used.
An alternate method of consolidating the layers of a substrate is to use a pitch or resin binder and press the materials under heat and pressure to form a preform. The preform is then typically batch carbonized to reduce its non-carbon content. The carbonized preform may then be die cut or machined to a desired shape for further densification.
The preform may be further densified by a chemical vapor deposition (CVD) process, e.g. with hydrocarbon gasses, by resin infiltration or by pitch infiltration and then carbonized yet again. These densification processes may be repeated until the desired density is attained. The preform may then be heat-treated to reorient the carbon atoms, which modifies the thermo-mechanical properties of the preform, machined if necessary, and the non friction surfaces treated with an anti-oxidant to form the finished carbon-carbon part.
It has been found that, for preforms containing pitch, if the preform is carbonized without undergoing a stabilization step, a significant amount of the liquid pitch runs out of the preform during carbonization. This limits the density of the resulting carbon-carbon preform. In an effort to address this problem, the present inventors have attempted to carbonize performs while they are contained in a restraint fixture. The use of such a restraint fixture substantially addresses the problem of run out, but may cause damage to the finished perform. This is because, while the preform and the restraint fixture both expand when heated during the carbonization process, the restraint fixture contracts to a greater extent that the preform as it cools and thus squeezes and sometimes damages the preform. It would therefore be desirable to provide a restraint fixture for quickly and efficiently producing preforms that reduces the need for a stabilization step and avoids pitch run out during carbonization.